In recent years, lithium-ion secondary batteries have become increasingly important as onboard power sources for vehicles, or as power sources for personal computers and portable terminals. In particular, expectations are being placed on lithium-ion secondary batteries, which are lightweight and can achieve a high energy density, as promising for use as high-output onboard power sources for vehicles. Such lithium-ion secondary batteries are configured so that, for example, a wound electrode member is formed in which a positive electrode and a negative electrode, with a separator interposed, is wound, and this wound electrode member and electrolyte are accommodated in a battery case. In this mode, if foreign matter intrudes into the wound electrode member, this foreign matter may penetrate and rupture the separator, and short-circuiting of the positive electrode and negative electrode or other phenomena may occur. So as to suppress such short-circuits, for example, in Patent Reference 1, formation of a porous insulating layer, comprising alumina, silica, titanium oxide, or similar, on the surface of the negative electrode active-material layer, is disclosed.
Although not technology having as a main objective suppression of short-circuits such as described above, Patent Reference 2 and Patent Reference 3 also disclose technologies for forming porous films on the surfaces of negative electrodes. For example, in Patent Reference 2, provision of a porous lithium film on the surface of at least one among the positive electrode and the negative electrode is disclosed. Such a porous lithium film is provided mainly with the objective of improving the battery discharge capacity. And, Patent Reference 3 discloses a lithium-ion secondary battery the negative electrode of which is pure lithium or a lithium alloy, and in which a porous insulating film is formed on the negative electrode surface opposing the positive electrode. The material for such a porous insulating film is TiN or Al2O3. Such a porous insulating film is provided mainly with the objective of preventing dendrite occurrence.    Patent Reference 1: Japanese Patent Application Laid-open No. 2005-183179    Patent Reference 2: Japanese Patent Application Laid-open No. 2002-231221    Patent Reference 3: Japanese Patent Application Laid-open No. 06-036800
In a mode in which a porous insulating layer comprising alumina, silica, titanium oxide or similar is formed on the surface of the active-material layer of the negative electrode as in Patent Reference 1, there has been a phenomenon in which the battery output drops markedly in low-temperature environments below the freezing point in particular, such as for example extremely cold environments of −30° C. or similar.